After getting the quest from Storn Crag-Strider, head to Tel Mithryn and seek out Neloth. After asking him about the Black Book, he will tell you that although the Black Book he holds has nothing to do with Miraak, he has located another in the nearby ruins of Nchardak. The Dwarven ruin is partially submerged, and only by using special control cubes can you control the water levels, and activate the steam mechanisms that hold the Black Book under guard.

Storn Crag-Strider will send you on a quest to learn more about the Black Books from a Telvanni wizard named Neloth. If you went to the Earth Stone after arriving in Solstheim you might remember him as the only Dark Elf not mind controlled and building a shrine. He now resides in his giant mushroom house in Tel Mithryn, so head there and take the cool magic elevator to speak with him. Ask him about the power of the Black Books and he will consent that while he does possess a Black Book, it is not related to Miraak. He does know of a Black Book on the island that is close by, the Dwarven Ruin and Archive known as Nchardak. Apparently the Dwemer knew of its power and locked it away in a reading room.

Follow Neloth toward Nchardak but be wary, as many Reavers have taken up residence outside the entrance. Neloth will explain that he magically sealed the entrance so that none of them would enter. Defeat the Reavers and enter the reading room. The Black Book will be just out of reach behind a glass panel. Neloth will explain that the unlocking mechanism is steam powered, and though his previous explorations he has deduced that by using certain Control Cubes, one can raise and lower the water in the submerged city, and activate the steam mechanisms. In order to do this, you will need five control cubes. Follow Neloth deeper into the ruins.

Neloth will demonstrate the control cubes power by lowering the water to the steam mechanisms. Since he lacks the other control cubes, follow him to find more. He will give you his control cube to unlock a nearby door. Enter the next room and grab the nearby control cube. If only the rest were so easy. Taking the cube will disable the traps so proceed onwards. Enemies will start to appear in the form of Dwemer constructs so don't let your guard down.

This next area is known as the Workshop. At the moment it looks pretty flooded. Lower the water level using the control cubes you picked up, and prepare for some constructs to come knocking. Ignore the door to the south, it leads to a temporary dead end. Instead focus on the door to the north. A control cube can be placed on the bottom floor to create stairs up to the north door. Head through the door and to the left to grab another control cube. Now gather up your control cubes to raise the water level and head through the south door. With the water level risen you can float to the top and onward. This next area has some angry Dwemer constructs so be careful they don't ambush you. Taking the control cube in this room will raise the water and activate a centurion behind you. Once he is dead, activate the control cube on the top level to lower a bridge to head back to the main room.

Now that you have more cubes, use two control cubes to lower the water as far as you can, which reveals a path below the steam mechanisms. Several constructs will ambush you at this point. Activate the control cube pedestal at the north door to enter the Aqueduct. In this area the pump activator is out of reach and you'll need to get all three bridges down. Head to the three control cube slots and use the middle slot, the right slot, the middle slot, and finally the left slot to lower all three bridges. Battle the newly awakened enemies and make your way to the pump activator. It will lower the water, allowing you to make your way to the last control cube. Use the pedestal to unlock the bottom floor door, and enter an area with a few nasty spinning blade traps. Hug the wall and strafe the traps to safely reach the other door but beware the poison dart traps as you pick up the last control cube.

Now that you have all five control cubes, return to the Great Chamber. Using one control cube to lower the water to the steam mechanisms, place all four control cubes with their mechanisms. Do raise the water level back up or the steam mechanisms will flood and shut off. Return to the reading room and press the button to raise the Black Book. Read it to complete the quest and begin the quest The Gardener of Men.